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  Water pressure gauge, temperature gauge, or both?

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Author Topic:   Water pressure gauge, temperature gauge, or both?
PMUCCIOLO posted 07-08-2003 01:15 PM ET (US)   Profile for PMUCCIOLO  
We've always had temperature gauges on our boats. Is a water pressure gauge a good addition to the temperature gauge?
kingfish posted 07-08-2003 01:31 PM ET (US)     Profile for kingfish  Send Email to kingfish     

Opinion here, not science...

I have a pressure gauge, which I'm happy with and understand gives me additional information on the state of my impellor, but I don't have a temp gauge, and wish I did. I think they're both important, but do as I say, not as I do...


Jarhead posted 07-08-2003 03:28 PM ET (US)     Profile for Jarhead  Send Email to Jarhead     
I, like kingfish, have only the pressure guage.

However I'm giving serious consideration to adding a temp guage as well.

The good thing about a temp guage is it should stay constant so any veriation is easily noticed.

I guess it really boils down to what makes you feel the most comfortable.

lhg posted 07-08-2003 04:39 PM ET (US)     Profile for lhg    
I have both gauges on mine, and refer to them occasionally, but not continuously. These gauges are best for showing signs of impending needed repairs and for peace of mind. I am convinced that the water telltale and the overheat warning horn (if your engine has it) are still the best for getting your attention.

One day while happily out cruising along at 30 MPH, not paying any attention to the gauges, the overheat warning horn went off. Turning around to look at the engine, I quickly saw no telltale and a lot of steam. So we shut down and discovered we had picked up a plastic bag over the water intakes. The overheat warning horn, not the gauges, saved the engine.

For a larger V-6 two stroke, I think the cylinder head temp gauge is more valuable, unless you are running at elevated transom heights. Then you need both.

Recent discussion here indicates that for the Merc/Yamaha 75-115 4-strokes, both manufacturers are recommending the water pressure gauges instead, however.

cdnturbo posted 07-08-2003 05:48 PM ET (US)     Profile for cdnturbo  Send Email to cdnturbo     
I have both on my boat . I find that to me the pressure guage is more useful because you can see how your impeller is working or not.
But in the end the alarm is probably the most important because you don't want to constantly be watching the guages.
My last boat had twin 165 merc i/o engines that were constantly having problems, so I'm in the habit of watching my guages.
SSCH posted 07-08-2003 07:31 PM ET (US)     Profile for SSCH  Send Email to SSCH     
Here's another reason the water pressure gauge is the more important of the two. You can watch the "pop valve" that controls flow over the plane between the cylinder banks. When the valve opens, you will see the pressure drop to normal running pressures. If the gauge shows too high a pressure, you're not getting cooling to the part of the engine that sits between those cylinder banks. You can see this well before the horn goes off. On Yamaha engines the pop valve can be repaired for a few dollars.
dgp posted 07-08-2003 07:38 PM ET (US)     Profile for dgp  Send Email to dgp     
You can watch the "pop valve" that controls flow over the plane between the cylinder banks. When the valve opens, you will see the pressure drop to normal running pressures.


SSCH posted 07-09-2003 12:33 AM ET (US)     Profile for SSCH  Send Email to SSCH     

On V4 and V6 Yamaha engines there is a spring loaded valve that opens at about 3500 RPM or really when the water pressure reaches a certain point. That valve allows additional cooling water to flow over the metal plate between the cylinder banks. If this valve sticks the engine will over heat. When it opens, it actually drops the water pressure in the cylinder head since another exit point is created for the cooling water. I think this cooling technique is used in most V engine configurations, but I'm certain about the Yamahas since I have replaced this part on two of them.

So, the message is that you can watch the water pressure as it builds to the proper level and then continue to watch as the valve opens and lowers the pressure. It is possible to see a great deal of the cooling system functioning just by learning how to read that one gauge.

DADDYSSHRINKII posted 07-13-2003 11:13 PM ET (US)     Profile for DADDYSSHRINKII  Send Email to DADDYSSHRINKII     
I have been running outboards for well over 30 yrs. I have a pressure and temp gauges on both banks of my V-4 engine.
They have all come in handy on occasion.
Jerry Townsend posted 07-13-2003 11:37 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
This topic has been addressed previously - probably about a year ago.

When you come down to the nuts and bolts of the subject - the temperature is much, much more important than the pressure. The reason is that the temperature is a feedback signal whereas the pressure is really a feed-forward signal. That is, the temperature guage tells the operator the result of the coolant flow.

One can have all the presure in the world and if that pressure is not producing any flow around the cylinder walls or if that flow is not removing heat from the cylinders, you have a problem.

In short, a pressure guage tells you that you have pressure - and nothing more. The temperature guage tells you that you have flow and that flow is extracting heat from the cylinders. These are the parameters of importance!

There is no question - the temperature is, by far, the most important. ------ Jerry/Idaho

lhg posted 07-16-2003 02:54 PM ET (US)     Profile for lhg    
I just had a cooling water problem, although not so severe an overheat problem that the engine could not still be run, with one of my Mercury V-6 200 HP EFI's. The experience confirms to me, at least, that with a big 2-stroke V-6, you need BOTH Temp and Water Pressure gauges, which I do have.

In getting the problem fixed ($140 total) I learned a lot, which may be of help to others here. Although this is Mercury specific information, from what SSCH has correctly indicated above with Yamaha, it applys in principle to them also, although the design & placement sounds different. However, from what Kingfish has indicated on his OMC V-6 overheat problem, the OMC system may not be as complicated as the Merc or Yamaha design. Not sure on that.

What we are talking about here is the "water pressure relief poppit valve assembly" (about 14 pieces including gaskets). Mine had gone bad, and had to be replaced, as mentioned above. When I asked my Dealer what this thing was, which I had never heard of before, I learned the following.

First my symtoms. Working properly, at 30 mph cruising speed and 3000 RPM, the engines normally show about 6 PSI water pressure and temp in the 1/8 - 1/4 quadrant of the C-H range. Well, at this speed, the other engine was showing 10 PSI and temp in the 3/4-7/8 quadrant of C-H. So, I'm thinking, this is strange, great water pressure but it's running very warm. Then I notice cooling water (not hot at all) dripping out the engine pan and running down the midshaft, clearly where no water is supposed to be flowing. Several of us at the Door County Rendezvous this past weekend could not figure what in the world was wrong, and why water was venting out the side of the block, which we could see.

What was wrong was a diaphram in the "water pressure relief assembly" had ruptured, letting water escape out the side of the lower block, and not activating the poppit valve. So what is this system? The venting detail is intentional in case the assembly fails, and part of the design.

Evidently, big V-6's need to be quite warm at idle and below 2000 RPM, to achieve smooth operation. Kingfish mentioned this as information from OMC also. Then when on plane, they need the full impeller produced water for the higher RPM cooling. So Mercury accomplishes this by routing the cooling water through themostats ONLY at idle and slow speeds, which are set for higher operating temp. Then, as the engine speeds up, the "water pressure relief assembly (poppit valve) opens up under the increased impeller pressure and routes the water to BYPASS the themostats completely and give the engines the full cooling water charge. At this point, the PSI gauge shows the pressure drop and the themostats are bypassed, and the temp gauge shows the cylinder head temp drop. On Mercurys, this relief valve system is on the bottom, center, of the starboard side of the block, and a little hard to get to. Some ignition components have to be removed for access. So in my case, with the relief valve not functioning properly, water pressure running though the themostat restricted passages was very high as RPM increased, but adequate cooling water volume was not getting through.

Now that I know how this cooling system works, I can clearly see the need for BOTH gauges in tandem, and now understand why boats properly set up with V-6's have both. But the operator needs to be informed about this type of cooling system to be able to take advantage of what the gauges are telling you. For 14 years I did not know this, since I never had a problem like this before. I hope others here can learn from this experience, particularly if you have a Mercury or Yamaha V-6.

Older, and smaller engines may not have, or need, this type of sophisticated cooling system. My 1985 Mercury in-line 6's do not, and don't even have thermostats at all.

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